With progress towards miniaturization and large scale integration of semiconductor devices, the line width of a transistor has become minute. Because of this, hot carrier effects are generated within very small transistors. Hot carrier effects are increased when the transistor channel length is made shorter, but the externally applied voltage is not made proportionately lower, causing an increase in the electric field (volts/meter) accross the channel. The horizontal electric field in the channel is largely concentrated toward the drain end. Hot carrier effects generated by the large electric field cause deterioration of electrical characteristics in the channel near the drain region. Electron/hole pairs are generated by hot carriers in the transistor. Holes leak out toward the substrate. Electrons migrate towards the gate, and become trapped at the lower portion of a gate oxide layer or a spacer, affecting the threshold voltage.
To overcome hot carrier effects, a transistor may have an LDD (lightly doped drain) structure. The implanted ion concentration of the source/drain region formed within the substrate is low near the edges of the gate electrode. In contrast, the ion concentration is high at the center of the source/drain region, creating a graded junction to reduce a voltage gradient, and therefore the strength of the electric field.
However, with the relentless trend towards miniaturization, the channel length has become ever shorter, and the LDD structure has failed to completely control hot carrier effects. The dopant at the LDD region may diffuse into the channel. A high electric field is created between the channel edge and the drain. The hot carrier phenomenon again substantially degrades the performance of the transistor.
Impurities in the source and the drain diffuse to the side portions of the channel during the operation of the transistor, and induce a punch through effect. A special ion implantation process to prevent punch-through may be attempted, but this complicates the manufacturing process. Also, when it is difficult to control the channel length and ion concentration due to ion diffusion and hot carriers, the threshold voltage also becomes difficult to control.
A solution to the above-described problems is disclosed in Korean Patent Laid-open No. 2001-64434. A transistor with a trench-type gate electrode structure, in which the bottom surface of the gate electrode is buried in the substrate between spacers. A gate oxide layer with a concave profile is provided along the side portion and bottom portion of the gate electrode. However, the gate is partially buried while also rising high above silicon substrate. Problems still occur when the device is further miniaturized.